Comment 1 for bug 1276421

Without more context it's hard so tell, but this looks like it might be a hierarchy issue.

zope.component.getUtility() uses the "current site" to look up utilities, so it might well be that the global registry has one MailHost instance registered as a utility (which has smtp_queue=False), and your site has another instance registered (which has smtp_queue=True), and then somewhere there's a mixup about what the current site should be when looking up the utility.

But as I said, that's just a wild guess.